The present invention relates to the field of imaging in general, and more particularly, to the field of ultrasound imaging.
It is known to use imaging methods to investigate the mechanical properties of tissues. In particular, it is known to evaluate the mechanical properties of tissue using shear wave characteristics of the tissue. For example, some of the tissue properties which can be examined using shear waves include the speed at which shear waves travel in the tissue, the attenuation that the shear waves exhibit while propagating in the tissue, and the associated wavelengths of the shear waves.
As discussed in A. Sarvazyan et al. entitled xe2x80x9cShear wave elasticity imaging: A new ultrasonic technology of medical diagnostics,xe2x80x9d Ultrasound Med. Biol., 24(9):1419-1435, 1998 (hereinafter xe2x80x9cSarvazyanxe2x80x9d), shear waves are generated by focussing low frequency ultrasound waves on a focal point to provide point shear wave sources. These types of shear waves are also discussed, for example, in U.S. Pat. No. 5,810,731 to Sarvazyan et al. entitled Method and Apparatus for Elasticity Imaging Using Remotely Induced Shear Wave.
Embodiments according to the present invention can provide methods, systems and computer program products for imaging using virtual extended shear wave sources. Pursuant to these embodiments, ultrasound energy can be transmitted into tissue in a first direction to provide a virtual extended shear wave source. The virtual extended shear wave source can generate an extended shear wave that propagates in a second direction substantially orthogonal to the first direction to cause movement in the first direction of tissue that is offset from the virtual extended shear wave source in the second direction.
The movement (or displacement) of the tissue caused by the extended shear wave can be used to determine properties or images of the tissue. For example, in some embodiments according to the present invention, the displacements can be used to generate images of mean shear wave velocity, images of mean shear wave attenuation, images of differences in shear wave velocities and attenuations as a function of extended shear waves propagating in different directions or angles through the tissue, images of mean shear wave wavelength, images of differences in shear wave wavelength, and the to like.
In some embodiments according to the present invention, the ultrasound energy is transmitted by an ultrasound transducer array having an associated focal point, wherein the extended shear wave is located on at least one of a far side and a near side of the focal point relative to the ultrasound transducer array.
In some embodiments according to the present invention, the virtual extended shear wave source can be a transmit ultrasound beam having an amplitude sufficient to generate a trackable extended shear wave. In some embodiments according to the present invention, the transmitted ultrasound energy causes the tissue along the first direction to displace in a range between about 0.1 xcexcm and about 300 xcexcm.
In some embodiments according to the present invention, the virtual extended shear wave source can have an associated frequency in a range between about 1 MHz and about 25 MHz.
In some embodiments according to the present invention, transmitting can include transmitting ultrasound energy from an ultrasound transducer array having an associated focal point, wherein a portion of the virtual extended shear wave source extends at least a portion of a distance between the ultrasound transducer array and the focal point.
In some embodiments according to the present invention, the transmitting can include transmitting first ultrasound energy in the first direction to provide a first virtual extended shear wave source that generates a first extended shear wave. Furthermore, second ultrasound energy can be transmitted into the tissue in a third direction to provide a second virtual extended shear wave source, wherein the second virtual extended shear wave source generates a second extended shear wave that propagates substantially orthogonal to the third direction to cause movement in the third direction of tissue that is offset from the second virtual extended shear wave source.
In some embodiments according to the present invention, the virtual extended shear wave can be a first virtual extended shear wave and the transmitted ultrasound energy can be steered into the tissue in a third direction of transmission, that is different than the first, to provide a second virtual extended shear wave source in the tissue.
In some embodiments according to the present invention, a sub-aperture of an ultrasound transducer array used to transmit the ultrasound energy can be changed to provide a second virtual extended shear wave source in the tissue in a third direction.
In some embodiments according to the present invention, the steering can be provided by applying a phased excitation to an ultrasound transducer to steer the transmitted ultrasound energy in the third direction.
In some embodiments according to the present invention, the tracking can be provided by tracking the movement of the offset tissue in response to the extended shear wave using receive mode parallel processing. In some embodiments according to the present invention, the movement of the offset tissue in response to the extended shear wave can be tracked using a second modality.
In some embodiments according to the present invention, tissue can be tracked to obtain first position data associated with tissue. Ultrasound energy can be transmitted into the tissue in a first direction to provide a virtual extended shear wave source that generates an extended shear wave that propagates in a second direction substantially orthogonal to the first direction to cause movement in the first direction of tissue that is offset from the virtual extended shear wave source in the second direction. The offset tissue can be tracked to obtain second position data. A displacement map can be determined based on the first and second position data and an image of the tissue can be generated based on the displacement map.